Quite often industrial operations require simultaneous storage of a liquid in both relatively hot and relatively cold forms. It is common to use separate tanks for this purpose. One tank stores the hot liquid and the other tank stores the cold liquid.
To minimize capital investment, it has been previously proposed to store the same liquid at dual temperatures in one tank and to rely on specific gravity to maintain a hot volume above a lower cold volume. See Schmitt et al U.S. Pat. No. 4,315,404 in which a tank stores hot water above cold water, the cold water being used in peak shaving in an electrical generating plant.
Not only is cold water, or some other liquid, often stored for cooling but, in addition, it is necessary for various hot liquids to be stored to preserve and conserve thermal energy. Owing to a frequent mismatch between production and utilization of such energy, a need exists for its storage. For most practical applications, the storage temperatures could range from 75.degree. F. (24.degree. C.) to about 1050.degree. F. (566.degree. C.).
Thermal energy can be stored in the form of a hot liquid in a single tank if the liquid is to be discarded after its thermal energy is removed. However, if the liquid is to be reheated after its thermal energy is removed, it must be suitably stored for reuse. A separate or cold tank can be used for this purpose but this is expensive. Alternatively, a thermocline storage tank such as described above with the hotter liquid stored on top and the colder liquid at the bottom of the tank, can be used. Although such a tank is useful, particularly with water, there is nevertheless some undesirable mixing between the hot and cold layers when water is fed to or removed from the tank. Furthermore, other liquids otherwise suitable for storing thermal energy do not maintain a sharp thermocline so that effective separation of hot and cold layers is not always possible. A need accordingly exists for alternative means for storing a liquid at dual temperatures and variable volumes.